Single molecule measurements with photoelectron emission microscopy

In this research, variable wavelength photoelectron emission microscopy (PEEM) with tunable UV light from the Duke University free electron laser is applied to image single fibrinogen molecules adsorbed onto n -type silicon surfaces. High resolution PEEM images (∼10 nm) are obtained with photon energies from 4 to 6 eV. Wavelength-dependent image sequences are analyzed to determine the photoionization spectrum and the photoelectron emission threshold of individual molecules. The experimental data are fitted using temperature dependent Fowler law, square-root law, and cube-root law. The details of the theoretical models are discussed. The square-root and cube-root fittings reveal the ionization threshold of 5.0 eV for fibrinogen adsorbed onto n -type silicon, while temperature dependent Fowler law shows a threshold of 4.9 eV. The accuracy of the measurements is calculated to be ±0.2 eV. The authors conclude that no significant difference is observed from the three theoretical fitting approaches.